Support distinct types for Event fields

Add support for indexed (and non-indexed) Event fields types that are distinct `ValueType` or `SmallByteArray`. For example,
```nim
type
  DistinctAlias = distinct array[32, byte]
  MyEvent = object of Event
    a {.indexed.}: DistinctAlias
    b: DistinctAlias # also allowed for non-indexed fields

## The below funcs generally need to be included for ABI
## encoding/decoding purposes when implementing distinct types.

func toArray(value: DistinctAlias): array[32, byte] =
  array[32, byte](value)

func encode*(encoder: var AbiEncoder, value: DistinctAlias) =
  encoder.write(value.toArray)

func decode*(decoder: var AbiDecoder,
             T: type DistinctAlias): ?!T =
  let d = ?decoder.read(type array[32, byte])
  success DistinctAlias(d)
```

Readme.md

@@ -106,6 +106,35 @@ type Transfer = object of Event
 Notice that `Transfer` inherits from `Event`, and that some event parameters are
 marked with `{.indexed.}` to match the definition in Solidity.
 
+Note that valid types of indexed parameters are:
+```nim
+uint8 | uint16 | uint32 | uint64 | UInt256 | UInt128 |
+int8 | int16 | int32 | int64 | Int256 | Int128 |
+bool | Address | array[ 1..32, byte]
+```
+Distinct types of valid types are also supported for indexed fields, eg:
+```nim
+type
+  DistinctAlias = distinct array[32, byte]
+  MyEvent = object of Event
+    a {.indexed.}: DistinctAlias
+    b: DistinctAlias # also allowed for non-indexed fields
+
+## The below funcs generally need to be included for ABI
+## encoding/decoding purposes when implementing distinct types.
+
+func toArray(value: DistinctAlias): array[32, byte] =
+  array[32, byte](value)
+
+func encode*(encoder: var AbiEncoder, value: DistinctAlias) =
+  encoder.write(value.toArray)
+
+func decode*(decoder: var AbiDecoder,
+             T: type DistinctAlias): ?!T =
+  let d = ?decoder.read(type array[32, byte])
+  success DistinctAlias(d)
+```
+
 You can now subscribe to Transfer events by calling `subscribe` on the contract
 instance.
 

ethers/events.nim

@@ -1,4 +1,5 @@
 import std/macros
+import std/typetraits
 import pkg/contractabi
 import ./basics
 import ./provider
@@ -40,7 +41,10 @@ func decode*[E: Event](_: type E, data: seq[byte], topics: seq[Topic]): ?!E =
     if field.hasCustomPragma(indexed):
       if i >= topics.len:
         return failure "indexed event parameter not found"
-      if typeof(field) is ValueType or typeof(field) is SmallByteArray:
+      if typeof(field) is ValueType or
-        field = ?AbiDecoder.decode(@(topics[i]), typeof(field))
+         typeof(field) is SmallByteArray or
+         typeof(field).distinctBase is ValueType or
+         typeof(field).distinctBase is SmallByteArray:
+          field = ?AbiDecoder.decode(@(topics[i]), typeof(field))
       inc i
   success event

testmodule/testEvents.nim

@@ -3,6 +3,26 @@ import pkg/ethers
 import pkg/contractabi
 import ./examples
 
+## Define outside the scope of the suite to allow for exporting
+## To use custom distinct types, these procs will generally need
+## to be defined in the application code anyway, to support ABI
+## encoding/decoding
+type
+  DistinctAlias = distinct array[32, byte]
+
+proc `==`*(x, y: DistinctAlias): bool {.borrow.}
+
+func toArray(value: DistinctAlias): array[32, byte] =
+  array[32, byte](value)
+
+func encode*(encoder: var AbiEncoder, value: DistinctAlias) =
+  encoder.write(value.toArray)
+
+func decode*(decoder: var AbiDecoder,
+             T: type DistinctAlias): ?!T =
+  let d = ?decoder.read(type array[32, byte])
+  success DistinctAlias(d)
+
 suite "Events":
 
   type
@@ -25,6 +45,9 @@ suite "Events":
       d {.indexed.}: seq[byte]
       e {.indexed.}: (Address, UInt256)
       f {.indexed.}: array[33, byte]
+    IndexedWithDistinctType = object of Event
+      a {.indexed.}: DistinctAlias
+      b: DistinctAlias
 
   proc example(_: type SimpleEvent): SimpleEvent =
     SimpleEvent(
@@ -47,6 +70,14 @@ suite "Events":
       e: array[32, byte].example
     )
 
+  proc example(_: type IndexedWithDistinctType): IndexedWithDistinctType =
+    IndexedWithDistinctType(
+      a: DistinctAlias(array[32, byte].example)
+    )
+
+  func encode(_: type AbiEncoder, value: DistinctAlias): seq[byte] =
+    @(value.toArray)
+
   func encode[T](_: type Topic, value: T): Topic =
     let encoded = AbiEncoder.encode(value)
     result[0..<Topic.len] = encoded[0..<Topic.len]
@@ -71,6 +102,14 @@ suite "Events":
     let data = AbiEncoder.encode( (event.a, event.c) )
     check IndexedEvent.decode(data, topics) == success event
 
+  test "decodes indexed fields with distinct types":
+    let event = IndexedWithDistinctType.example
+    var topics: seq[Topic]
+    topics.add Topic.default
+    topics.add Topic.encode(event.a)
+    let data = AbiEncoder.encode( (event.b,) )
+    check IndexedWithDistinctType.decode(data, topics) == success event
+
   test "fails when data is incomplete":
     let event = SimpleEvent.example
     let invalid = AbiEncoder.encode( (event.a,) )